As is well known, household refrigerators (and here the term "refrigerators" is intended to include refrigerators with or without a freezing compartment, as well as freezers of either the upright or chest type) employ a closed-cycle expansion-compression refrigeration cycle, in which heat is abstracted from the interior of the refrigerator by a refrigerant in liquid phase and at low pressure, to vaporize the refrigerant, and the vaporized refrigerant is then compressed and cooled prior to reliquefaction upon expansion.
In household refrigerators, the cooling of the compressed gaseous refrigerant is effected in an air-cooled condenser. This is wasteful of energy, particularly when air conditioning units are in use for cooling the home. Moreover, the air-cooled condenser coils tend to become covered with dust and to lose efficiency and must be periodically cleaned. Still further, the interior space of the refrigerator is reduced by the need to provide thicker walls and greater insulation, so as to reduce the load on the condenser and hence to reduce the loss of energy at the condenser. Still further, a highly efficient compressor motor must be used in such a system, and this raises the initial cost of the refrigerator.
It is also well known that household water heaters are heated by an external energy source, e.g. natural gas or electricity. These heaters are ordinarily in the form of an upright vertically elongated tank. The water in the tank is ordinarily characterized by a high degree of stratification: that is, the water at the top will be relatively hot and that at the bottom will be relatively cold, and so there will be a sharp temperature gradient from top to bottom in the tank. The cold water supply is introduced into the bottom of the tank and hot water for use is removed from the top of the tank. Often, however, the supply of hot water is insufficient, as the heating means for the water in the tank can heat to an acceptable temperature level only a certain quantity of water per unit time.
It is also known that, in a household, peak demand for refrigeration usually precedes by a predetermined amount of time, the peak demand for hot water. For example, the household refrigerator door is frequently open and the heat ingress to the refrigerator is greatest, during the preparation of a meal. Immediately following the meal, however, the demand for hot water may rise to its peak, as the dishes are washed. Thus, maximum energy loss via the household refrigerator ordinarily precedes maximum energy loss via the household hot water heater.
Finally, it is known that in most household refrigerators, the compressor is operating about 80% of the time; whilst household hot water heaters are obliged to cycle on every so often, to replace heat lost to the ambient through the insulation.